A composite is a material in which two or more constituents are combined to result in a material which has properties different from either component. Typical composites are from materials in which one of the components has very high strength and modulus and the other has high ductility. Their properties generally follow the rule of mixtures. For example, if elastic modulus is the property of interest, the elastic modulus of the composite is approximately the weighted sum of the elastic modulus of the constituents.
Metal matrix composites provide a relatively new way of strengthening metals. Liquid-phase fabrication methods are particularly suited for the cost effective production of metal matrix composite parts using fibers. Uniform fiber distributions of the fibers can generally be achieved with little porosity in the matrix. However, the contact of liquid-phase metal with fibers often induces interfacial reactions.
The terms "aluminum" and "aluminum alloy" are used interchangeably herein to describe pure aluminum and all aluminum base alloys.
The term "liquid-phase metal" is used herein to describe all fluid and semi-fluid phases in which the metal was not completely solidified. The term includes metal slurry and semi-solid phases.
The term "liquid crystal polymer fiber" is used herein to describe all polymer fibers formed from liquid crystal polymers. The term includes but is not limited to fibers made from "KEVLAR", "XYDAR", "NOMEX", "VECTRA", polybenzobisthiazole, polybenzobisimidazole, polybenzobisoxazole, polyamide, polyamide-imides, polyester-imides, polysiloxanes and copolymers, polysiloxane-carborane, polyphosphazenes, polyquinoxalines, poly etheretherketones, and polyether sulfones.
The term "high thermal stability polymer fiber" is used herein to describe all polymer fibers that can be heated above 250.degree. C. without melting or decomposing. The term includes liquid crystal polymer fibers.
Metal matrix composites are reinforced with either ceramic or graphite fibers. Ceramic or graphite fibers are used because it has always been thought that they are most suited to withstand the processing temperatures needed to bring the metal component to its molten or liquid phase without degradation of the fiber.
Fiber surface coatings are applied to the surface of some ceramic and graphite fibers with the aim of modifying the fiber surface characteristic so as to prevent deterioration in fiber stiffness and strength at elevated fabrication temperatures and to enhance the fiber/matrix wettability and adhesion. Fiber/matrix wettability and adhesion is extremely important since good bonding between the fiber and matrix is crucial to obtaining the maximum final strength of the metal matrix composite. However, known surface coatings and treatments for ceramic and graphite fibers are expensive and have not proven to be reliable. In addition, the ceramic or graphite fibers used are brittle and are sensitive to handling. This has further increased the cost of fabricating a metal matrix composite.
There exists a need for a metal matrix composite that is formed from a fiber that does not require surface coatings to withstand the high temperatures associated with liquid-phase metal fabrication methods. Heretofore, polymer fibers have not been used in metal matrix composites because of thermal degradation of the polymers.
The principal object of the present invention is to provide a liquid-phase fabrication method and metal matrix composite which does not suffer from the limitations of prior metal matrix composites.
Another object of the present invention is to provide a liquid-phase fabrication method and metal matrix composite which utilizes polymer fibers and relatively short processing times which will not cause the polymer fibers to degrade.
Still another object of the present invention is to provide a liquid-phase fabrication method and metal matrix composite which does not require surface treatment or coating of the fibers prior to liquid-phase fabrication of the composite to increase the fiber/matrix wettability and adhesion and/or to reduce brittle compound reactions between fibers and metal.
Yet another object of the present invention is to provide a liquid-phase fabrication method and metal matrix composite which has a low void fraction.
A further object of the present invention is to provide a metal matrix composite that is light in weight.
Additional objects and advantages of the invention will be more fully understood and appreciated with reference to the following description.